Melting and refining metals



Oct. 15, 1957 M. DAVIS EIAL MELTING AND REFINING METALS Filed Dec. 21, 1956 2,809,905 MELTING AND REFINING IVIETALS Michael Davis and Reginald Frank Lever, Baldock, England, assignors to National Research Development (Iorporation, London, England, a British corporation Application December 21, 1956, Serial No. 629,960

9 Claims. (Cl. 148-1) This invention is concerned with the melting and refining of metals, or other materials of a refractory nature, by what is termed the floating zone method, in which a rod of material to be refined has a small portion of its length heated to melting point and the molten zone is moved along the length of the rod by relative movement between the rod and the source of heat, so that impurities having markedly different solubilities in the liquid and solid phases respectively can be made to concentrate towards an end of the rod, leaving the major portion of the rod with increased purity. This method is advantageous in avoiding contamination, since a crucible is not needed if the rod is mounted vertically and the molten zone is kept sufficiently narrow for the surface tension of the molten portion to be effective in retaining it in place. The method has been applied to the purification of silicon, using localised heating, either by high frequency induction or by radiation from a short cylindrical refractory metal electric heater, but both of these ways of heating are very wasteful of power (a radiation heater needs to be very much hotter than the molten part of the rod), and it is diflicult to keep the molten zone suiiiciently narrow. -In the case of some very refractory metals, it may not be possible to concentrate the heat sufiiciently to be able to melt even a small zone.

One object of the present invention is to provide for the melting and refining in this way of materials, such as tungsten and tantalum and even of refractory non-metals, which would otherwise not be capable of being so treated because 'of the lack of sufficiently powerful radiation heaters or for other reasons.

A further object is to reduce the power requirements for melting and treating refractory materials by the floating zone method, whilst another object is to ensure fine control of the temperature and dimensional limits of the molten zone.

Still another object is to provide for thorough outgassing whilst the zone refining is being carried out, of any materials containing occluded gases or which release gases when heated.

These and other objects are achieved according to this invention by mounting a rod of the material to be treated within an evacuated enclosure and bombarding a narrow zone of the rod with electrons focussed on the zone from an electron gun' so that the electrons are accelerated on to said zone and concentrated heating and melting occur without causing the melt to run from the bar.

Thus, the rod can be mounted as the anode of an electron discharge system consisting of a cathode surrounding the rod with focussing electrodes to concentrate an annular electron stream from the cathode on to the rod which is maintained electrically positive to the cathode to accelerate electrons on to the rodto produce high concentration of heat over a very narrow sharply defined zone.

The traversing of the molten zone along the rod can be done either by fixing the annular cathode and moving atent the rod through it, or by fixing the rod and traversing the cathode and focus electrodes along it.

The accompanying drawing illustrates, in cut away perspective View, one suitable form of apparatus for carrying out the invention.

It is most convenient to have the specimen being treated and the major portion of the apparatus at earth potential and to maintain the cathode at high negative voltage. The whole electrode system is hung from a stainless steel metal cover 1 so that adjustments may easily be made before being enclosed in the container 2 or bombardment chamber which is a flanged tube of Pyrex glass. The cover lfits on the top flange of the chamber 2 and ahermetic seal at the joint is ensured by flexible seal rings 3. After closure the chamber 2 can be evacuated, through pipe 4, by vacuum pump 5 which produces and maintains a high vacuum inside the chamber 2.

The specimen of material to be treated is in the form of a rod 6 which is held at its ends in V grooves in mild steel blocks 7 and 7 by springs 8 which allow for expansion and contraction of the specimen during operation. The blocks 7 and 7 are recessed to provide better location of the specimen and to reduce thermal contact. The lower block 7 is attached to the ends of support rods 9 on which the upper block 7 is slidable for adjustment by means of screw 10. The rods 9 are carried by the bridge 11 which is attached to the lower end of the operatingrod 12 which is slidable up and down through the vacuum ring seal 13 in the cover 1 to enable the specimen rod to be moved longitudinally through the electron source. The longitudinal movement of the operating rod 12 to move the specimen rod 6 is effected by means of the mechanism 14 driven by electric motor 15 so as to give a traversing speed of between 1 and 2% cm./minute.

The source of electrons for bombardment heating of the rod 6 is a cathode loop 16 preferably of tungsten wire as its emissive properties are not harmed by exposure to air between successive operations and during loading of the vacuum chamber. However, in some cases it may be advantageous in avoiding contamination of the specimen by volatile fragments of the cathode, to use a cathode loop of the same metal as that of which the rod 6 is made whenever that is possible. The cathode loop 16, with its focussing shields 17, is supported by blocks 18 on the lower ends of two fixed metal rods 19 which con: duct the heater current to the loop 16 and extend through insulating bushes 20 in the cover 1 to terminate in terminals 21 for the attachment of heater supply leads from a power transformer secondary which is also connected to the negative side of a power supply unit 22 having its positive pole grounded. An earthing terminal 23 is provided on cover 1 so that the rod 6 is thereby held at a potential highly positive to the cathode loop 16 and thus acts as the anode of the electron discharge system and attracts to itself with high velocity the electrons emitted from the cathode and focussed on a narrow zone around the surface of the rod 6 which is thereby heated to incandescence and subsequently melted by the electron bombardment. The focussing shields 17 are of metal such as molybdenum or tantalum. and are normally connected directly to the cathode, but they can be at a lower or higher potential than the cathode to control the degree of focussing necessary.

in operation, the cover plate 1 and the parts depending from it are removed from the chamber 2 and the specimen rod 6 is mounted with its ends held by the springs 8 in the blocks 7 and 7'. It is better to have the rod 6 cut in half, with the cut ends aligned and touching, so as to avoid any danger of the setting up process causing stresses which might cause the two parts of the rod to become misaligned when melting takes place. After checking that the cathode and focussing plates are aligned to clear the specimen rod throughout the length of its travel, the assembly is placed in the bombardment chamber which is then evacuated. When the internal pressure is sufficiently reduced the abutting ends of the two halves of the rod 6 are brought opposite the cathode loop 16 and the H. T. supply from source 22 is switched on. The emission current from the cathode is thereby caused to bombard the rod at the narrow zone surrounded by the cathode loop and this zone becomes red hot and finally molten and can be made to travel along the rod by moving the latter endwise by the traversing mechanism 14, 15.

The electron bombardment heating, besides requiring less power than radiation or eddy current heating, permits very accurate control of the width of the molten zone so that there is never any danger of the zone becoming so large that the surface tension of the molten material fails to keep the melt from running out. The size of molten zone which can be attained without the melt running from the bar, depends of course on the surface tension and density of the liquid metal. The largest diameter of rods which can be so treated are about 1 cm. dia. in the case of silicon and about 0.5 cm. dia. in the case of rods of tungsten. The length of the molten zone which can be supported by surface tension of the melt is about the same as the diameter of the rod. There is no theoretical lower limit of diameter, but there are practical limitations, although specimens as low as 2 mm. diameter have been zone refined.

As an alternative form of the apparatus illustrated, the supporting structure for the rod 6 can be fixed and the cathode loop 16 with its focussing shields 17 can be made to traverse alongithe stationary rod; in that case the blocks 18 are a one-piece Mycalex insulating plate fixed to a screwed nut on a threaded rod or lead screw between the two fixed rods 19. The threaded rod projects above the top cover 1 through a vacuum seal, and rotation of the threaded rod by an external drive moves the electron source up and down sliding on the rods 19 as guides. In this case the heater current is carried to the heater loop by flexible metallic tapes from the terminals 21.

The material to be treated, which forms the rod 6, must of course be a substance which does not undergo extensive physical or chemical changes when it is heated to melting, that is, it must be stable when molten. Also, since the treatment takes place in an evacuated chamber, the material should have negligible vapourpressure at its melting point (i. e. below about -100 microns of mercury).

Provided the material is stable when molten and has low vapour pressure, the method of the invention is applicable, not only to the refining and treatment of metals, but also to semi-conductor materials, and even to normally insulating materials, such as alumina for instance, since nearly all insulators become conductive when hot or molten, and only at the start of the process is it necessary to make them slightly conductive, as by marking them or coating them with graphite. Alternatively, an electron gun, similar in principle to that employed in many cathode ray tubes may be used. In this case the electrons are accelerated by and through an or refining of (a) body-centered cubic lattice materials, such as molybdenum, tungsten, and iron, in which minute traces of carbon, oxygen or nitrogen can cause brittleness, especially at low temperatures; (b) reactive high temperature metals whose ductility can be seriously impaired by amounts of carbon, oxygen, or nitrogen exceeding 0.1%, such as beryllium, titanium, zirconium, hafnium, vanadium, columbium, tantalum, thorium, uranium and rare earth elements. This invention enables the amount of such impurities to be reduced to harmless proportions, in consequence of zone refining or of vacuum fusion.

Besides purification by zone refining in which the impurities dissolve in the molten zone, the material being treated in the way above described is also further purified by evaporation of volatile impurities and by very thorough outgassing as the result of fusion of the material in a vacuum.

The evaporation of the very volatile parts of the material as a result of vacuum fusion can be utilised for vacuum deposition of the volatile products on to other articles which are to be coated with or have deposited on them such volatile products by placing the articles inside the evacuated chamber- 2 before the process is started in operation and locating them in such positions that the evaporate is driven on to them.

The outgassing may be considerable with some specimens when the material first melts, and the outgassing may be so violent as to disrupt the specimen if too much of it is melted at one time. The sudden surge in bombardment current which occurs during excessive outgassing may cause the zone length to increase and become unstable, so that continuous control of the cathode emission current is necessary during outgassing. The procedure for zone melting a material which may evolve large quantities of gas during melting is therefore to do the heating in stages; first the bombarding current is controlled so that only the outer layer of the bombarded zone becomes molten and the released gas carries out droplets of molten material as a shower of sparks. The current is then held at that value and a first pass of the molten strip along the rod is made at about 1 cm./min. and then successive additional passes are repeated with gradually increasing values of bombarding current until no more gas evolution with showers of sparks occurs. At this time outgassing is complete and the molten zone extends right through the specimen. Repeated passes of the molten zone along the rod results in rapid increase of grain size and finally in the production of a single crystal of the material forming the treated rod.

Single crystal production in this way is only possible in refractory metals which do not undergo phase changes on cooling, but the method of the invention has enabled large single crystals, from 2 mm. to 1 cm. diameter and up to 16 cms. long, of the following refractory metals to be obtained, namely Tungsten Platinum Tantalum Nickel Rhenium Silicon Molybdenum Ruthenium Vanadium In some cases it may be necessary to start the crystal growth by means of a seed crystal to ensure the desired orientation.

The invention is particularly useful for zone refining the metals listed above, but it can be used for treating many other metals and also other materials such as alumina, which has been successfully zone melted by the method of the invention.

The method has the great advantages thatthe heat input is highly concentrated, making it possible to floating zone melttungsten and the other refractory metals where power requirements would otherwise be prohibitive.

Control of the power is extremely simple and the method is applicable to small diameter specimens.

We claim:

1. A method of refining a refractory material which is stable when molten and has a negligible vapour pressure at its melting point but contains an impurity of higher solubility in the material when molten than in the material in its solid state, which method comprises the steps of supporting a solid rod of the material in an evacuable enclosure containing an electron gun, evacuating said enclosure, heating the rod of material in the evacuated closure by electron bombardment from said gun confined to a limited zone of the rod to melt the material at said zone without causing the melt to run from the rod, and traversing said rod and said gun relative to one another to move the melted zone along the rod to collect in said zone the impurities soluble therein and thereby to transfer them to an end portion of the rod.

2. A method as claimed in claim 1 in which the passing of the molten zone along the rod is repeated to increase the material grain size and produce single crystals of said material.

3. A method of outgassing and refining a refractory material which is stable when molten and has a negligible vapour pressure at its melting point but contains an impurity of solubility ditferent in the solid and liquid phases of said material, which method comprises the steps of supporting a solid rod of the material in an evacuable chamber containing an electron gun, evacuating said chamber, heating a limited zone of the surface of said rod by electron bombardment from said gun, traversing said rod and said gun relative to one another to move the heated zone along the rod, repeating the passes of the heated zone along the rod at successively increasing intensities of bombardment until the rod is gas free and heated right through to melt a portion of said rod without causing the melt to run from it, and traversing the rod and gun relatively to one another to pass the molten portion along the rod to collect in said zone the impurities soluble therein.

4. A method of producing large single crystals from refractory metal which is stable when molten, has negligible vapour pressure at its melting'point and does not change phase on cooling, comprising the steps of mounting a rod of said metal within an evacuated enclosure containing an annular electron gun surrounding said rod, heating a limited portion of said rod by electron bombardment from said gun to melt a zone of the rod so limited that the molten zone is held to the unmelted part of the rod by natural forces in the material, and repeatedly passing the molten zone along the rod by traversing the rod and gun relatively to one another in the direction of the length of the rod.

5. The method of outgassing, refining, and converting into a single crystal a rod of a refractory metal which is stable when molten, of negligible vapour pressure at its melting point, and does not change phase on cooling, comprising mounting a rod of said metal upright within an exacuated enclosure as the anode of electron discharge system consisting of a ring cathode around the rod with focussing means to concentrate a stream of electrons from the cathode on to the rod to melt by electron bombardment a narrow zone thereof which is so limited that it is held to the unmelted remainder of the rod by natural forces in the metal of the rod, and traversing said molten zone along the rod by relative movement, longitudinally of the rod, between the rod and the cathode.

6. Metallurgical apparatus comprising a rigid chamber which can be hermetically sealed, a vacuum pump for evacuating said chamber, supports within said chamber for mounting by its ends a rod of refractory material, an electron gun operative to bombard with electrons a limited area of said rod and means for moving said rod and said gun relatively to one another longitudinally of said rod.

7. Metallurgical apparatus comprising a chamber which can be hermetically sealed, pumping means for continuously evacuating said chamber, supports inside the chamber to grip the ends of a refractory metal rod and hold it upright within the chamber, an electron gun adapted to bombard with electrons a limited area of said rod, and mechanism for moving said rod and said gun relatively to one another longitudinally of said rod.

8. Metallurgical apparatus comprising a rigid chamber which can be hermetically sealed, pumping means for continuously evacuating said chamber, supports inside the chamber to grip the ends of a refractory metal rod and hold it upright within the chamber, a ring cathode around said rod inside the chamber, supply leads for conducting heating current to the cathode to energise it to electron emission, focussing means associated with the cathode to direct electrons from the cathode on to a limited zone of said rod, a voltage supply source to maintain said rod positive to the cathode to accelerate the electrons on to said zone, and drive mechanism for traversing the cathode and gun relatively to one another longitudinally of said rod.

9. Metallurgical apparatus comprising a rigid chamber which can be hermetically sealed, a pump for evacuating said chamber, vertically movable supports inside the chamber to hold the ends of a refractory metal rod and hold it upright, guides for said supports, a fixed ring cathode around said rod inside the chamber, supply leads to conduct heating current to said cathode, focus shields to direct the electron emission from the cathode on to a limited zone of said rod, a voltage source to maintain the rod positive to the cathode, and drive mechanism for drawing the rod longitudinally through the cathode and its focus shields.

References Cited in the file of this patent UNITED STATES PATENTS 2,739,088 Pfann Mar. 20, 1956 2,771,568 Steigerwald Nov. 20, 1956 2,778,926 Schneider Jan. 22, 1957 2,792,317 Davis May 14, 1957 

1. A METHOD OF REFINING A REFRACTORY MATERIAL WHICH IS STABLE WHEN MOLTEN AND HAS A NEGLIGIBLE VAPOUR PRESSURE AT ITS MELTING POINT BUT CONTAINS AN IMPURITY OF HIGHER SOLUBILITY IN THE MATERIAL WHEN MOLTEN THAN IN THE MATERIAL IN ITS SOLID STATE, WHICH METHOD COMPRISES THE STEPS OF SUPPORTING A SOLID ROD OF THE MATERIAL IN AN EVACUABLE ENCLOSURE, CONTAINING AN ELECTRON GUN, EVACUATING SAID ENCLOSURE, HEATING THE ROAD OF MATERIAL IN THE EVACUATED CLOSURE BY ELECTRON BOMBARDMENT FROM SAID GUN CONFINED TO A LIMITED ZONE OF THE ROD TO MELT THE MATERIAL AT SAID ZONE WITHOUT CAUSING THE MELT TO RUN FROM THE ROD, AND TRAVERSING SAID ROD AND SAID GUN RELATIVE TO ONE ANOTHER TO MOVE THE MELTED ZONE ALONG THE ROD TO COLLECT IN SAID ZONE THE IMPURITIES SOLUBLE THEREIN AND THEREBY TO TRNASFER THEM TO AN END PORTION OF THE ROD. 